1. Field of the Invention
The present invention relates to a method for controlling a switching power unit, more particularly to a control method that is properly applied to a switching power unit being composed of an AC/DC converter circuit which includes a power-factor correction (PFC) unit and a DC/DC converter unit.
2. Description of the Related Art
For digital apparatuses or domestic amusement equipments such as a laptop computer, a liquid crystal television, a plasma television, a game instrument and the like, a switching power unit that is composed of an AC/DC converter in which to improve a power-factor has been applied. In general, the switching power unit comprises a full-wave rectifier bridge, a booster-type power-factor correction (PFC) unit and a DC/DC converter unit.
For the DC/DC converter unit, a flyback type converter, a forward type converter, a current resonance (LLC) type converter, etc. can be named. Here, when a highly effective power supply is needed, the current resonance type converter has been widely applied.
In FIG. 8, a half-bridge type switching power unit is disclosed. See Japanese Patent Application Laid-Open No. 2008-283818 (hereinafter referred to as the “Patent Document”). This half-bridge type switching power unit is composed of a full-wave rectifier bridge 18, a power-factor correction unit 20, and a half-bridge type current resonance converter 30.
In the circuit structure of the half-bridge type switching power unit, the power-factor correction unit 20 has an active filter including an inductor 21, a diode 22 and a switch element 23 and a smoothing capacitor 26 between the full-wave rectifier bridge 18 and two switching elements 31, 32 provided on the input side of the current resonance converter 30.
Further, in the current resonance converter 30, a series resonant circuit (a resonant capacitor 33 and a resonant inductor 34) is connected between the intermediate point of a series circuit composed of the switching elements 31, 32 and the primary winding of a transformer TF. Current that flows toward the secondary winding of the transformer TF is rectified and smoothed by rectifier diodes 35, 36 and a capacitor 37 so as to obtain predetermined output voltages.
In this circuit, by alternatively turning ON and OFF the switching elements 31, 32 through a frequency controller 38, high frequency voltages that have been switched are applied to both ends of the primary winding of the transformer TF. The high frequency voltages are then output to the secondary side of the transformer TF thereby being converted into the output voltages of direct current.
This switching power unit can convert the current resonance converter 30 in a high efficient condition; however, the switching power unit is a multi-level circuit structure composed of 3 elements: the full-wave rectifier bridge 18; the power-factor correction unit 20; and the current resonance converter 30. Accordingly, the overall efficiency of the unit will be generally reduced by approximately 85 to 90%.
In the above condition, in a prior application (Japanese Patent Application No. 2010-85394) the present inventors have invented a full-bridge type switching power unit by eliminating a full-wave rectifier bridge and to improve conversion efficiency by sharing a power-factor correction unit and the switching elements of a current resonance converter unit.
FIG. 9 is a block diagram that shows an AC/DC converter circuit 1 which constitutes the main portion of the switching power unit discussed hereinabove. The AC/DC converter circuit 1 of FIG. 9 is composed of a power-factor correction (PFC) unit 2 and a current resonance (LLC resonance) type DC/DC converter unit 3 (hereinafter referred to as the “current resonance converter unit 3”). In the power-factor correction unit 2, the series circuit of first/second diodes D1, D2 and the series circuit of first/second switching elements Q1/Q2 are connected to each other in parallel while a booster inductor L1 and an AC source Vac is connected to each other in series between these series circuits. Further, a first smoothing capacitor Ci is connected to the both ends of the series circuit of the first/second diodes D1, D2 in parallel relative to the first/second switching elements Q1, Q2.
In the block diagram of FIG. 9, the booster inductor L1 has one end that is connected to the intermediate point of the first and second diodes D1, D2, and has the other end that is connected to one end of the AC source Vac. However, it would be possible that the AC source Vac and the booster inductor L1 may be arranged in an inversed manner, or either a resonant capacitor Cr or a resonant inductor Lr of a series resonant circuit 6, or both of them may be connected between the intermediate point of a third/fourth switching elements Q3, Q4 and the primary winding of a high frequency transformer TF. Further, the resonant inductor Lr may be replaced by a leakage inductance of the high frequency transformer TF.
In the current resonance converter unit 3, the power-factor correction unit 2 and the first/second switching elements Q1, Q2 are shared with each other while the series circuit of the first/second switching elements Q1, Q2 and the series circuit of the third/fourth switching elements Q3, Q4 are connected to each other in parallel. Accordingly, a full-bridge circuit 5 that is composed of four switching elements (that is, the switching elements Q1 to Q4) is formed. Here, the first/third switching elements Q1, Q3 are arranged on a high voltage side while the second/forth switching elements Q2, Q4 are arranged on a low voltage side.
In the current resonance converter unit 3, at the subsequent stage of the full-bridge circuit 5, the series resonant circuit 6 including the resonant inductor Lr and the resonant capacitor Cr is arranged on the primary side of the high frequency transformer TF. On the other hand, a rectifier circuit that is composed of rectifier diodes D3, D4 and a second smoothing capacitor Co is provided at the secondary side of the high frequency transformer TF. As is shown in FIG. 9, the series resonant circuit 6 and the rectifier circuit are arranged to sandwich the high frequency transformer TF. The series resonant circuit 6 is connected in series to the primary winding of the high frequency transformer TF, and also connected to the intermediate point of the first/second switching elements Q1, Q2 as well as the intermediate point of the third/fourth switching elements Q3, Q4.
The AC/DC converter circuit 1 is composed of: a PFC voltage detector 10 that detects voltages at both ends of the first smoothing capacitor Ci of the power-factor correction unit 2 (referred to as the “PFC voltage” when appropriate); an output voltage detector 11 that detects the output voltage of the AC/DC converter circuit 1 (that is, voltages at both ends of the second smoothing capacitor Co); a polar detector 13 that detects the polarity of the AC source Vac; and a switching controller 12 that controls ON and OFF of the first to fourth switching elements Q1 to Q4. Output singles from the PFC voltage detector 10, the output voltage detector 11 and the polar detector 13 are input to the switching controller 12. Based on the output signals, the switching controller 12 generates pulse signals that drive the first to fourth switching elements Q1 to Q4. The pulse signals are then output to each of the switching elements. Normally, the first to fourth switching elements Q1 to Q4 are structured by MOSFET. In this case, the pulse signals that drive each of the switching elements Q1 to Q4 are gate drive signals.
In this switching power unit, the power-factor correction unit 2 constitutes a dual-boost converter circuit that controls PFC voltages based on the on-duty of the first switching element Q1 in the negative half cycle of the AC source Vac (in this period, the third switching element Q3 should turn ON and OFF in order to have the same on-duty with the first switching element Q1). On the other hand, in the positive half cycle of the AC source Vac, the dual-boost converter circuit controls the PFC voltages based on the on-duty of the second switching element Q2 (in this period, the fourth switching element Q4 should turn ON and OFF in order to have the same on-duty with the second switching element Q2).
The switching power unit is thus allowed to individually perform 1) the PWM control of PFC voltages in the power-factor correction unit 2 as discussed above; and 2) the output voltage controls of the current resonance converter 3 by changing the switching cycles of the full-bridge circuit 5. Further, since the power-factor correction unit 2 and the current resonance converter 3 share the first and second switching elements Q1, Q2, it becomes possible to structure the full-bridge switching power unit with no full-wave rectifier bridge thereby contributing to cost reduction and structure simplification by reducing a number of parts. Still further, since it can reduce switching losses and avoid multi-stages in the circuit structure, a highly efficient switching power unit with advanced power-factors can be achieved.
Here, in the AC/DC converter circuit 1 as shown in FIG. 9, as discussed hereinabove, switching elements that turn ON and OFF to obtain on-duty for controlling the PFC voltages (referred to as the “control gate” when appropriate) are switched between the pair of first/third switching elements Q1, Q3 and the pair of second/fourth switching elements Q2, Q4 based on the polar switching of the AC source Vac. However, when the control gate is simply switched as discussed above, the following problems may occur. Details are explained with reference to FIG. 10.
FIG. 10 exemplifies a case that polarity is switched from the negative half cycle to the positive half cycle of the AC source Vac, and shows normally expected drive controls at the switching elements Q1 to Q4 which switch the control gate. In the drive controls, a switching point M at the control gate indicates the starting point of a next new one cycle following a completed one cycle (of a switching cycle) that may include a point where the polarity of the AC source Vac has been switched. In this case, the polarity of the AC source Vac may have been switched at any time during the precedently completed one cycle right before the switching point M of the new cycle (meaning that at any time during a period I). Here, during the negative half cycle of the AC source Vac, gate drive signals that are output to the first/third switching elements Q1, Q3 are simply switched from the switching point M so as to output each of the second/fourth switching elements Q2, Q4. Accordingly, the control gate is switched from the first/third switching elements Q1, Q3 to the second/fourth switching elements Q2, Q4.
In this method, as shown in FIG. 10, around the switching point M, high frequency voltages that have been applied to the primary winding of the high frequency transformer TF become asymmetrical to each other in a positive-negative relation. More specifically, during the positive and the negative half cycles of the AC source Vac, the full-bridge circuit 5 should alternately repeat the following condition in every half cycle of the switching cycle: a first condition where the first/fourth switching elements Q1, Q4 are turned ON (the second/third switching elements Q2, Q3 are turned OFF); and a second condition where the second/third switching elements Q2, Q3 are turned ON (the first/fourth switching elements Q1, Q4 are turned OFF). Accordingly, the high frequency voltages that are symmetrical in a positive-and-negative relation are normally applied to the primary winding of the high frequency transformer TF. However, immediately before and after the switching point M, the first condition repeats twice with no intervention of the second condition. As a result, homopolar resonance voltages and homopolar resonance current are consecutively input twice to the high frequency transformer TF, meaning that the positive-negative asymmetry of the high frequency voltages occurs. Further, in case that polarity is switched from the positive half cycle to the negative half cycle of the AC source Vac, as long as the identical driver controls are based, the same problem should occur.
Thus, when the polarity of the AC source Vac is switched, large excitation current will temporarily flow to the high frequency transformer TF. Since this large excitation current appears in a cycle twice as much as the frequency of the AC source Vac, core vibrations of the high frequency transformer TF are generated at audio frequency bands thereby causing noises from the high frequency transformer TF.